This invention relates to the field of hollow fiber production, especially to hollow fibers for use in asymmetric fluid separation membranes.
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor. This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. Large rates of permeation also allow the membrane to be operated at lower pressures, which improves cost-effectiveness and reduces wear and tear.
Highly porous membranes tend to be very permeable, but do a poor job of separating fluids into their components. Less porous, dense membranes can be more selective, but at the cost off reduced throughput.
Asymmetric membranes generally have a thin, dense separation layer and a less dense, more permeable layer. These asymmetric membranes are often chosen for separation applications because they may provide a good combination of permeation and separation. Hollow fibers having walls that are primarily porous and a dense skin or layer on the inner or outer surface of the wall can be used to make such membranes.
A limiting factor in the separation efficiency of asymmetric hollow fibers is the presence of defects or holes in the dense separation layer that nonselectively increase the permeability of fluids, thus decreasing the separation efficiency of the fiber.
U.S. Pat. Nos. 4,717,393 and 4,717,394, both issued to Hayes, disclose semipermeable polyimide gas separation membranes.
U.S. Pat. No. 4,838,900 issued to Hayes teaches aromatic polyimide gas separation membranes.
U.S. Pat. No. 4,871,494 issued to Kesting, et al. describes a process for forming asymmetric gas separation membranes having graded density skins. This process comprises dissolving a hydrophobic polymer in a Lewis acid:base solvent system wherein the Hildebrand parameters of the solvent species and the polymer are within less than 1.5, creating a dope from this solution, forming the dope into an appropriate shape, coagulating the dope, desolvating, washing and drying. According to the patent, the resultant membrane has increased free volume as evidenced by a glass transition temperature greater than the bulk glass transition temperature of the polymer.
U.S. Pat. No. 4,881,954 issued to Bikson, et al. describes permeable membranes for enhanced gas separation in which an asymmetric porous support is coated with a separation layer. The support is made asymmetric prior to coating to provide mechanical stability and a uniform support surface.
U.S. Pat. No. Re. 30,351 issued to Hoehn, et al. discloses aromatic polyimide, polyester, and polyamide gas separation membranes.
Pending U.S. patent application Ser. No. 686,739, the disclosure of which is herein incorporated by reference, describes asymmetric fluoropolymer hollow fibers made by a dry-jet wet-spinning process in which there is an air gap between the hollow fiber die and the coagulation bath.